Bumps for chip scale packaging including under bump metal structures with different diameters

ABSTRACT

A chip scale semiconductor device comprises a semiconductor die, a first bump and a second bump. The first bump having a first diameter and a first height is formed on an outer region of the semiconductor die. A second bump having a second diameter and a second height is formed on an inner region of the semiconductor die. The second diameter is greater than the first diameter while the second height is the same as the first height. By changing the shape of the bump, the stress and strain can be redistributed through the bump. As a result, the thermal cycling reliability of the chip scale semiconductor device is improved.

This application is a continuation of U.S. patent application Ser. No.13/294,859, entitled “Bumps for Chip Scale Packaging,” filed on Nov. 11,2011, which application is incorporated herein by reference.

BACKGROUND

The semiconductor industry has experienced rapid growth due tocontinuous improvements in the integration density of a variety ofelectronic components (e.g., transistors, diodes, resistors, capacitors,etc.). For the most part, this improvement in integration density hascome from repeated reductions in minimum feature size, which allows morecomponents to be integrated into a given area. As the demand for evensmaller electronic devices has grown recently, there has grown a needfor smaller and more creative packaging techniques of semiconductordies.

As semiconductor technologies evolve, chip-scale or chip-size packagingbased semiconductor devices have emerged as an effective alternative tofurther reduce the physical size of a semiconductor chip. In achip-scale packaging based semiconductor device, the packaging isgenerated on the die with contacts provided by a variety of bumps. Muchhigher density can be achieved by employing chip-scale packaging basedsemiconductor devices. Furthermore, chip-scale packaging basedsemiconductor devices can achieve smaller form factors,cost-effectiveness, increased performance and lower power consumption.

A chip-scale packaging based semiconductor device may comprise aplurality of solder balls formed on a plurality of under bump metal(UBM) openings of a semiconductor die. The solder balls may be formed oftin and lead. Prior to a reflow process, the semiconductor device ispicked and placed on a printed circuit board (PCB) after alignment. As aresult, the plurality of solder balls on the chip-scale packaging basedsemiconductor device are aligned with the corresponding solder pads onthe PCB board. By employing a hot air flow and appropriate pressure, thesolder balls are heated and then melted so as to connect thesemiconductor device with the PCB board. The chip-scale packagingtechnology has some advantages. One advantageous feature of chip-scalepackaging is that chip-scale packaging techniques may reduce fabricationcosts. Another advantageous feature of chip-scale packaging basedmulti-chip semiconductor devices is that parasitic losses are reduced byemploying bumps sandwiched between a semiconductor device and a PCBboard.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a top view and a cross sectional view of a bumpstructure having a chip scale packaging feature in accordance with anembodiment;

FIG. 2 illustrates a top view and a cross sectional view of a bumpstructure having a chip scale packaging feature in accordance withanother embodiment; and

FIG. 3 illustrates a top view and a cross sectional view of a bumpstructure having a chip scale packaging feature in accordance with yetanother embodiment.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the disclosure,and do not limit the scope of the disclosure.

The present disclosure will be described with respect to embodiments ina specific context, a bump design technique for a chip scale package.The disclosure may also be applied, however, to a variety of packages ofthe semiconductor industry.

Referring initially to FIG. 1, a top view and a cross sectional view ofa bump structure having a chip scale packaging feature is illustrated inaccordance with an embodiment. As shown in FIG. 1, the bump structure isformed on a semiconductor die 100. The semiconductor die 100 comprises asubstrate 192. The substrate 192 may be a silicon substrate.Alternatively, the substrate 192 may be a silicon-on-insulatorsubstrate. The substrate 192 may further comprise a variety ofelectrical circuits (not shown). The electrical circuits formed on thesubstrate 192 may be any type of circuitry suitable for a particularapplication.

In accordance with an embodiment, the electrical circuits may includevarious n-type metal-oxide semiconductor (NMOS) and/or p-typemetal-oxide semiconductor (PMOS) devices such as transistors,capacitors, resistors, diodes, photo-diodes, fuses and the like. Theelectrical circuits may be interconnected to perform one or morefunctions. The functions may include memory structures, processingstructures, sensors, amplifiers, power distribution, input/outputcircuitry or the like. One of ordinary skill in the art will appreciatethat the above examples are provided for illustrative purposes only tofurther explain applications of the present disclosure and are not meantto limit the present disclosure in any manner.

An interlayer dielectric layer 182 is formed on top of the substrate192. The interlayer dielectric layer 182 may be formed, for example, ofa low-K dielectric material, such as silicon oxide. The interlayerdielectric layer 182 may be formed by any suitable method known in theart, such as spinning, chemical vapor deposition (CVD) and plasmaenhanced chemical vapor deposition (PECVD). It should also be noted thatone skilled in the art will recognize that the interlayer dielectriclayer 182 may further comprise a plurality of dielectric layers.

A bottom metallization layer 172 and a top metallization layer 152 areformed over the interlayer dielectric layer 182. As shown in FIG. 1, thebottom metallization layer 172 comprises a first metal line 174.Likewise, the top metallization layer 152 comprises a second metal line162. Metal lines 174 and 162 are formed of metal materials such ascopper or copper alloys and the like. It should be noted while FIG. 1shows the bottom metallization layer 172 and the top metallization layer152, one skilled in the art will recognize that one or more inter-metaldielectric layers (not shown) and the associated metallization layers(not shown) are formed between the bottom metallization layer 172 andthe top metallization layer 152. Generally, the one or more inter-metaldielectric layers and the associated metallization layers are used tointerconnect the electrical circuits in the substrate 192 to each otherand to provide an external electrical connection.

A passivation layer 142 is formed on top of the top metallization layer152. In accordance with an embodiment, the passivation layer 142 isformed of non-organic materials such as un-doped silicate glass, siliconnitride, silicon oxide and the like. An aluminum pad 160 may be formedon top of the passivation layer 142. Furthermore, the aluminum pad 160may be connected to the top metal line 162 through a via hole 164. Inaccordance with an embodiment, the via hole 164 is fully filled withmetallic materials such as copper, copper alloys, aluminum, silver, goldand any combinations thereof. The via hole 164 may be formed by suitabletechniques such as CVD. Alternatively, the via hole 164 may formed bysputtering, electroplating and the like.

A first polymer layer 132 is formed on top of the passivation layer 142.The first polymer layer 132 is made of polymer materials such as epoxy,polyimide and the like. The first polymer layer 132 may be made by anysuitable method known in the art such as spin coating. A redistributionlayer 166 is formed on the first polymer layer 132. As shown in FIG. 1,the redistribution layer 166 connects the aluminum pad 160 with the topsurface of the semiconductor die 100. More particularly, theredistribution layer 166 provides a conductive path between the metallines (e.g., metal line 162) and the top surface of the semiconductordie 100 (e.g., UBM 116).

A second polymer layer 122 is formed on top of the first polymer layer132. As shown in FIG. 1, both the redistribution layer 166 and theredistribution layer 164 are embedded in the second polymer layer 122.The redistribution layer 166 and the redistribution layer 164 are formedof metal materials such as aluminum, aluminum alloys, copper or copperalloys and the like. The second polymer layer 122 is patterned to form aplurality of openings. Furthermore, various under bump metal (UBM)structures (e.g., UBM 116) are formed on top of the openings. The UBMstructures (e.g., UBM 116) are employed to connect the redistributionlayers (e.g., redistribution layer 166) with various input and outputterminals (e.g., bumps 106 and 104). Each UBM structure may furthercomprise a variety of sub-layers such as a seed layer (not shown), anadhesion layer (not shown) and/or the like. The UBM structures may beformed by any suitable techniques such as evaporation, electroplatingand the like.

Bumps 104 and 106 are formed on top of the UBM structures 114 and 116respectively. In accordance with an embodiment, the bumps 104 and 106are solder balls. Throughout the description, for simplicity, bumps 104and 106 are alternatively referred to as solder balls 104 and 106respectively. The solder balls 104 and 106 may be made of any ofsuitable materials. In accordance with an embodiment, the solder balls104 and 106 comprise SAC405. SAC405 comprises 95.5% Sn, 4.0% Ag and 0.5%Cu.

As shown in FIG. 1, after a reflow process, both solder balls 104 and106 are heated and subsequently melted so as to connect thesemiconductor die 100 with a printed circuit board (PCB) 102. It shouldbe noted that the PCB 102 may have a different thermal expansioncoefficient from the solder balls 104 and 106 as well as thesemiconductor die 100. As a result, such a different thermal expansioncoefficient may cause a relatively large stress and plastic strain atthe solder joint between the solder balls 104, 106 and the PCB 102.Furthermore, the large stress and plastic strain accumulated over aplurality of temperature cycles on the solder balls 104 and 106 may leadto cracks in the solder joint areas between the solder balls 104, 106and the PCB 102.

In order to redistribute the stress and strain described above evenlyacross the body of the solder balls (e.g., solder ball 104), solderballs on the top surface of a semiconductor die 100 may be configuredsuch that an inner region comprises large solder balls (e.g., solderball 106) and an outer region comprises small solder balls (e.g., solderball 104). A top view of the semiconductor die 100 illustrates a solderball layout in accordance with an embodiment. In the top view, thesemiconductor die 100 is of a horizontal length W1 and a vertical lengthW2.

On the top surface of the semiconductor die 100, there may be a varietyof solder balls formed on their corresponding UBM structures. As shownin FIG. 1, a plurality of small solder balls (e.g., solder ball 104) areformed on the outer region of the semiconductor die 100. Likewise, aplurality of large solder balls (e.g., solder ball 106) are formed onthe inner region of the semiconductor die 100. It should be noted thatin accordance with an embodiment, “a small solder ball” means itsdiameter is less than 90% of that of “a large solder ball.” In otherwords, when a large solder ball has a diameter of about 300 um, thecorresponding small solder ball has a diameter equal to or less than 270um. One advantageous feature of having small solder balls formed on theouter region of the semiconductor die 100 is that the small solder ballsallow the semiconductor die 100 to have a fine pitch package as well asaddition input and output terminals.

Furthermore, in accordance with an embodiment, the diameter of the smallsolder balls before a reflow process should be greater than the heightof the large solder balls after the reflow process. For example, priorto a reflow process, the diameters of a large solder ball and a smallsolder ball are 250 um and 225 um respectively. After a reflow process,both solder balls are melted and sandwiched between the top surface ofthe semiconductor die 100 and the PCB 102. The height of the largesolder ball after a reflow process is about 210.5 um. Therefore, bycontrolling the distance between the PCB 102 and the semiconductor die100, reliable solder joints formed by small solder balls can beachieved.

There may be a variety of ways to define the border between an innerregion and an outer region. In accordance with an embodiment, an outerregion comprises four edge regions. Each edge region has a width (e.g.,W₃ and W₄) approximately equal to or less than 20% of the correspondinglength (e.g., W₁ and W₂) of the semiconductor die 100. By employingdifferent solder balls on the top surface of the semiconductor die 100,after a reflow process, the outer bumps (e.g., solder ball 104) arethinner in comparison with their inner counterparts (e.g., solder ball106). As a result, the stress derived from the thermal expansiondifference between the PCB 102 and the semiconductor die 100 may beredistributed across the body of the solder ball 104 so that thepossibility of cracks may be reduced. One advantageous feature of havingsmall solder balls at the outer region of the semiconductor die 100 isthat the small solder balls help to redistribute the stress and strainso as to improve thermal cycling reliability. In accordance with anembodiment, the accumulative plastic strain during one cycle of thetemperature cycling test (TCT) at the solder joint between thesemiconductor die 100 and the PCB 102 can be reduced by 16%.

FIG. 2 illustrates a top view and a cross sectional view of a bumpstructure having a chip scale packaging feature in accordance withanother embodiment. The structure of the semiconductor die 200 shown inFIG. 2 is similar to the semiconductor die 100 shown in FIG. 1 exceptthat the UBM structures (e.g., UBM structure 216) of the semiconductordie 200 is different from the UBM structures (e.g., UBM structure 116)of the semiconductor die 100. As shown in the top view (solder balls areremoved in order to illustrate the UBM difference), a plurality of largeUBM structures (e.g., UBM structure 216) are employed in the outerregion of the semiconductor die 200. In contrast, a plurality of smallUBM structures are employed in the inner region. In accordance with anembodiment, the small UBM structures (e.g., UBM structure 214) has adiameter less than or equal to 90% of that of the large UBM structures(e.g., UBM structure 216).

In accordance with an embodiment, solder balls (e.g., solder 204 and206) having substantially identical size are formed on the UBMstructures having different diameters. As a result, the solder balls atdifferent regions may have different shapes after a reflow process. Moreparticularly, the solder ball formed on a large UBM structure isstretched during a reflow process in comparison with the solder ballformed on a small UBM structure. As shown in FIG. 2, the solder balls(e.g., solder ball 206) formed on the outer region may have an hourglassshape. In contrast, the solder balls (e.g., solder ball 204) formed onthe inner region may have a spherical shape. Such an hourglass shapedsolder column sandwiched between the semiconductor die 200 and the PCB102 helps to reduce the stress at the solder joint between thesemiconductor die 200 and the PCB 102.

FIG. 3 illustrates a top view and a cross sectional view of a bumpstructure having a chip scale packaging feature in accordance with yetanother embodiment. The structure of the semiconductor die 300 shown inFIG. 3 is similar to the semiconductor die 100 shown in FIG. 1 exceptthat the allocation of smaller solder balls of the semiconductor die 300is different from that of the semiconductor die 100. As shown in the topview, four small solder balls are formed on the four corners of theintegrated chip die 300. In contrast, large solder balls are formed inthe inner region. For similar reasons described above with respect toFIG. 1, the small solder balls of the semiconductor die 300 help toreduce the stress and strain at the solder joints between the smallsolder balls and the PCB 102 so as to reduce the possibility of cracksand improve thermal cycling reliability.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the present disclosure, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

What is claimed is:
 1. A device comprising: a semiconductor diecomprising a substrate, electrical circuits formed along a first side ofthe substrate and a redistribution layer over the first side of thesubstrate; a plurality of under bump metal structures arranged in rowsand columns on a center region of a top surface of the substrate; afirst under bump metal structure formed in a first region of thesemiconductor die, wherein the first under bump metal structure is of afirst diameter and a bottom of the first under bump metal structure isin direct contact with the redistribution layer, and wherein the firstunder bump metal structure is electrically connected to the electricalcircuits through a first metal line in a bottom metallization layer, asecond metal line in a top metallization layer, a via in a passivationlayer, an aluminum pad in a first polymer layer and the redistributionlayer; a second under bump metal structure formed in a second region ofthe semiconductor die, wherein the second under bump metal structure isof a second diameter and a bottom of the second under bump metalstructure is in direct contact with the redistribution layer, andwherein the second under bump metal structure and the first under bumpmetal structure are immediately adjacent to each other; a third underbump metal structure having a third diameter formed in the second regionof the semiconductor die, wherein the second under bump metal structureand the third under bump metal structure are in a same row andimmediately adjacent to each other, and wherein the third diameter isequal to the second diameter and the first diameter is greater than thesecond diameter; a fourth under bump metal structure having the firstdiameter, wherein the fourth under bump metal structure and the firstunder bump metal structure are in an outermost column of the pluralityof under bump metal structures and are immediately adjacent to eachother; a fifth under bump metal structure having the first diameter,wherein the fourth under bump metal structure and the fifth under bumpmetal structure are in an outermost row of the plurality of under bumpmetal structures and are immediately adjacent to each other, and whereina distance between the fourth under bump metal structure and the fifthunder bump metal structure is greater than a distance between the secondunder bump metal structure and the third under bump metal structure; afirst bump formed over the first under bump metal structure; and asecond bump formed over the second under bump metal structure, wherein atop surface of the first bump is level with a top surface of the secondbump, and wherein a first sidewall of an edge portion of the secondunder bump metal structure is covered by the second bump and a secondsidewall of the edge portion of the second under bump metal structure isnot covered by the second bump.
 2. The device of claim 1, wherein: thefirst region is an outer region of the semiconductor die; and the secondregion is an inner region of the semiconductor die.
 3. The device ofclaim 2, wherein: the outer region has a width approximately equal to orless than one third of a width of the inner region.
 4. The device ofclaim 2, wherein: the second diameter is less than or equal to 90% ofthe first diameter.
 5. The device of claim 1, wherein: the first bump isformed of solder; and the second bump is formed of solder.
 6. The deviceof claim 1, wherein: the first bump is of an hourglass shape.
 7. Thedevice of claim 1, wherein: the second bump has a diameter greater thana diameter of the first bump.
 8. A device comprising: a semiconductordie comprising a substrate, electrical circuits formed along a firstside of the substrate and a redistribution layer over the first side ofthe substrate, wherein the redistribution layer comprising aredistribution line; a first under bump metal structure formed adjacentto an edge of the semiconductor die and over the first side of thesubstrate, wherein the redistribution layer is connected between analuminum pad and a bottom of the first under bump metal structure, andwherein the first under bump metal structure is electrically connectedto the electrical circuits through a first metal line in a bottommetallization layer, a second metal line in a top metallization layer, avia in a passivation layer, the aluminum pad in a first polymer layerand the redistribution line; a second under bump metal structure formednot adjacent to the edge of the semiconductor die, wherein the firstunder bump metal structure has a diameter greater than a diameter of thesecond under bump metal structure; a first bump formed over the firstunder bump metal structure, wherein a sidewall of the first bumpcomprises a first slope portion, a vertical sidewall portion and asecond slope portion, and wherein an edge portion of the first underbump metal structure is not completely covered by the first bump, andwherein the edge portion of the first under bump metal structure extendsfrom an outermost edge of the redistribution line to an outermost edgeof the first under bump metal structure; and a second bump formed overthe second under bump metal structure, wherein the second bump is of aspherical shape, and wherein a top surface of the first bump is levelwith a top surface of the second bump, and wherein a first sidewall ofan edge portion of the second under bump metal structure is covered bythe second bump and a second sidewall of the edge portion of the secondunder bump metal structure is not covered by the second bump.
 9. Thedevice of claim 8, wherein: the second bump has a diameter greater thana diameter of the first bump.
 10. The device of claim 8, wherein: thefirst under bump metal structure is formed on an outer region of thesemiconductor die; and the second under bump metal structure is formedon an inner region of the semiconductor die.
 11. The device of claim 10,wherein: the outer region has a width approximately equal to or lessthan one third of a width of the inner region.
 12. A device comprising:a plurality of small under bump metal structures arranged in rows andcolumns on a center region of a top surface of a substrate, wherein theplurality of small under bump metal structures are of a same diameter; afirst row of large under bump metal structures between a first edge ofthe center region and a first edge of the substrate, wherein the firstrow comprises a first under bump metal structure and a second under bumpmetal structure adjacent to each other; a second row of large under bumpmetal structures between a second edge of the center region and a secondedge of the substrate, wherein the first edge and the second edge of thecenter region are on opposite sides of the center region; a first columnof large under bump metal structures between a third edge of the centerregion and a third edge of the substrate, wherein the first columncomprises the first under bump metal structure and a third under bumpmetal structure adjacent to each other; a second column of large underbump metal structures between a fourth edge of the center region and afourth edge of the substrate, wherein the third edge and the fourth edgeof the center region are on opposite sides of the center region, andwherein a distance between two immediately adjacent large under bumpmetal structures is greater than a distance between two immediatelyadjacent small under bump metal structures, and wherein each of thelarge under bump metal structures has a diameter greater than a diameterof each of the small under bump metal structures; a redistribution lineunder and in contact with a large under bump metal structure; a firstbump formed over the large under bump metal structure, wherein an edgeportion of the large under bump metal structure is not completelycovered by the first bump, and wherein the edge portion of the largeunder bump metal structure extends from an outermost edge of theredistribution line to an outermost edge of the large under bump metalstructure; and a second bump formed over a small under bump metalstructure, wherein a first sidewall of an edge portion of the smallunder bump metal structure is covered by the second bump and a secondsidewall of the edge portion of the small under bump metal structure isnot covered by the second bump.
 13. The device of claim 12, wherein: atop surface of the first bump is level with a top surface of the secondbump.
 14. The device of claim 12, wherein: the small under bump metalstructure is immediately next to the large under bump metal structure.15. The device of claim 12, wherein: the first under bump metalstructure is at a corner of the top surface of the substrate.
 16. Thedevice of claim 12, wherein: large under bump metal structures of thefirst row, the second row, the first column and the second column are ofa same diameter.
 17. The device of claim 12, wherein: the first bump isof an hourglass shape.
 18. The device of claim 12, wherein: the secondbump is of a spherical shape.